Method of making an electrical capacitor

ABSTRACT

An electrical capacitor according to the invention comprises a pair of electrodes and an electrically conductive material in contact therewith, at least one of the electrodes being of titanium-zirconium alloy, the electrode having an anodic dielectric oxide film thereon. The Ti-Zr electrode preferably comprises 40-90 atomic percent of Ti and 10-60 percent of Zr, and has a sintered porous structure. In one form, the electrically conductive material is MnO2; in another form, it is an aqueous solution of phosphoric acid, sulfuric acid, ammonium borate or potassium nitrite. A method of manufacture is also disclosed.

United States Patent Nakata et al.

[54] METHOD OF MAKING AN ELECTRICAL CAPACITOR Inventors: Koreaki Nakata, 20 Ohata-cho, Nishinomiya-shi, I-lyogo-ken; Yoshio Iida, 11-3 4-chome Fujishirodai, Suita-shi, Osaka-fu, both of Japan Filed: July 24, 1969 Appl. No 870,941

Foreign Application Priority Data Apr. 21, 1967 Japan ..42/25937 July '19, 1967 Japan ..42/46934 Related US. Application Data Division of Ser. No. 704,057, Feb. 8, 1968, abandoned.

'U.S. c1 ..29/570, 29/2542 Int. Cl. ..B01j 17/00, H01g9/00 Field of Search ..29/570, 25.42; 317/230 [56] References Cited UNITED STATES PATENTS 7/1967 Brown et a1. ..3l7/230 6/1967 Minami et al...., ..29/57O Sept. 12, 1972 3,321,678 5/1967 Dakin et al. ..3l7/23O 3,466,230 9/ l 969 Carithers ..204/38 3 ,244,946 4/ l 966 Hilton ..317/230 Primary Examiner-John F. Campbell Assistant Examiner-D. M. Heist Attorney-Wenderoth, Lind & Ponack [57] ABSTRACT An electrical capacitor according to the invention comprises a pair of electrodes and an electrically conductive material-in contact therewith, at least one of the electrodes being of titanium-zirconium alloy, the

electrode having an anodic dielectric oxide film thereon. The Ti-Zr electrode preferably comprises 40-90 atomic percent of Ti and 10-60 percent of Zr, and has a sintered porous structure. In one form, the electrically conductive material is MnO in another form, it is an aqueous solution of phosphoric acid, sulfuric acid, ammonium borate or potassium nitrite. A method of manufacture is also disclosed.

4 Claims, 2 Drawing Figures PATENTED SEP 12 m2 KDREMM NAKATR M10 YDSHlO IIBA. MMMv/MJIM INVENTORS METHOD OF MAKING AN ELECTRICAL CAPACITOR This is a division of Ser. No. 704,057, filed Feb. 8, I968, is now abandoned.

This invention relates to an electrical capacitor comprising an alloyof titanium and zirconium'and to a method for making the same and more particularly to a method for making an electrode having an anodic dielectric oxide film formed thereon. 1

Electrical capacitors, especially those of the electrolytic type, commonly employ metal electrodes on which a thin dielectric oxide coating has been formed. Heretofore, aluminum and tantalum have been commonly employed as capacitor electrode materials, and while each material has been found particularly useful under certain conditions, they have certain drawbacks. Aluminum is of light weight and superior in the ductility which facilitates making a thin foil. In addition, aluminum foil is easily increased in the surface area thereof by surface etching, and is less expensive than tantalum. Therefore, aluminum in a foil form has been used widely for making electrolytic capacitors of the wet type. On the other hand, tantalum forms a dielectric oxide film thereon which is superior in electrical properties, especially in the leakage current at high voltages and easily makes a porous body compared to aluminum. However, tantalum is more expensive than aluminum and is restricted in application.

Titanium has also been considered heretofore for possible use as a capacitor electrode material in view of the high dielectric constant of its oxide and other good properties such as corrosion resistance and low density. However, it has been difficult to make a dielectric oxide film on the surface of titanium which is superior in leakage current at high voltages. Such drawback of titanium prevents a wide use thereof for an electrode adapted to an electrolytic capacitor, in spite of its high dielectric constant and good sinterability. Especially, a porous body of titanium is known to be inferior in respect of leakage current when formed into an electrolytic capacitor of the solid type.

Therefore, an object of the present invention is to provide titanium alloy electrode characterized by a dielectric oxide film formed thereon having a low leakage current at high voltages.

Another object of the invention is to provide a method for making titanium alloy electrode characterized by a dielectric oxide film formed thereon having a low leakage current at high voltages.

A further object of the invention is to provide a method for making titanium alloy suitable for an electrode material capable of forming a dielectric oxide film thereon having a low leakage current at high voltages.

A further object of the titanium alloy electrolytic capacitor characterized by a low leakage current at high voltages.

A further object of the invention is to provide a titanium alloy electrolytic capacitor of the solid type characterized by a large capacitance and a low leakage current at high voltages.

These and other objects of the invention will be evident upon consideration of the following detailed description taken together with accompanying drawings wherein:

invention is to provide a FIG. 1 is a cross sectional view of an electrolytic capacitor of the wet type in accordance with the invention.

FIG. 2 is a cross sectional view of an electrolytic capacitor of the solid type in accordance with the invention.

Before proceeding with the detailed description of the novel electrode and its preparation method contemplated by the invention, a construction of an electrical capacitor comprising said electrode will be described with reference to FIGS. 1 and 2 of the drawings.

Referring to FIG. 1, reference character 30 indicates, as a whole, a so-called wet electrolytic capacitor comprising a container 4 serving as the cathode and containing an electrolyte 3 in which an anode l is immersed. Said anode 1 consists of titanium alloy according to the invention and is formed into a desired form and is anodically oxidized in a manner illustrated in detail hereinafter. In this embodiment, said anode 1 is a sintered body of titanium alloy prepared in a manner described hereinafter. A film-forming lead wire 2 is embedded in said sintered body 1 of titanium alloy and extends to an insulating plug 5 made of any insulating and water tight material such as rubber or resin. Said filmforming lead wire may be made of the same alloy, or of niobium, titanium, tantalum or other film-forming metal. Electric leads 6 and 7 made of any solderable metal such as copper or iron are attached to said filmforrning lead wire 2 and said container 4 in a per se well known method such as a welding method.

Referring to FIG. 2, reference character 40 indicates, as a whole, a so-called solid electrolytic capacitor comprising a base electrode 1 contemplated by the invention. Said base electrode 1 may be of any form such as plate, wire or sintered form prepared in a manner illustrated hereinafter. Said sintered body 1 has a film-forming lead wire 2 embedded therein. Said sintered body and film-forming lead wire are coated with oxide film 11 by being anodically oxidized in a manner illustrated hereinafter. Said film-forming lead wire 2 is made of the same alloy, or of niobium, titanium, tantalum or other film-forming metal. Said oxide fihn l 1 is coated with a layer 8 of a semi-conductive material in a per se well known method. A carbon film layer 9 is integrated onto said layer 8 of semi-conductive material by a per se conventional method such as a brushing method in which a carbon ink is applied by brushing. A silver paint 10 as counter electrode is applied to said carbon film layer 9. Electric leads 6 and 7 made of any solderable metal such as copper and nickel are attached in per se conventional manner to said film-forming lead wire 2 and said counter electrode 10, respectively.

It has been discovered according to the present invention that an alloy of titanium and zirconium can form a dielectric oxide film having a low leakage current at high voltages when anodically oxidized. Preferable and optimum compositions are as follows in accordance with the invention:

Zr to 60 to 50 Said alloy of titanium and zirconium can be made in a per se conventional and suitable manner, for example, by a vacuum melting of a mixture in a given composition or by sintering a mixture of a given composition in a non-reactive atmosphere such as in argon or under a reduced pressure less than 10* mm Hg at800 to 1,400 C similarly to conventional powder metallurgy technique.

Said alloy can be anodically oxidized in any aqueous solution per se well known in the art and is preferably anodically oxidized by employing a bath composition of aqueous solution consisting of 0.1 to 20 percent of phosphoric acid. Table 2 indicates the electrical properties of sintered bodies-which are obtained by heating mixtures of titanium particles of about microns and zirconium particles of about 4 microns in various compositions at 1,300 for 3 hours at a reduced pressure of 10' mm Hg of air and furnace-cooled. Soproduced sintered bodies have no porosity because relatively fine metal particles are employed, and are anodically oxidized in a bath composition of 10 percent by weight of phosphoric acid at a direct current voltage of 60 volts for minutes. Capacitance and dissipation factor are measured at 120 cps, employing a capacitance bridge. Leakage current is measured by immersing the anodically oxidized alloy into an aqueous solution of 10 percent by weight of phosphoric acid after application of a direct current voltage as large as two thirds of the formation voltage for about 5 minutes. The term leakage factor is expressed by leakage current divided by the capacitance and the measuring volt age.

A further discovery is that said alloy to be anodically oxidized is improved in leakage factor of the dielectric oxide film formed thereon when the alloy is heated in a non-reactive atmosphere such as argon and is cooled to room temperature of about C from a temperature of 500 to 1,000 C at a rate higher than 50 C per minute.

Table 3 shows the examples according to the invention. Alloy of titanium and zirconium in various compositions are prepared by heating mixtures of titanium particles and zirconium particles in various compositions in a way similar to that described above. So produced alloys are in the form of a sintered body hav ing no porosity and are heated at 1,000 C at a reduced pressure of about 10' mm Hg and cooled to room temperature of about 30 C at a rate of 50 C per minute. The cooled alloys are anodically oxidized in a bath composition comprising an aqueous solution of 10 percent by weight of phosphoric acid so as to form dielectric oxide films thereon. The electrical properties of dielectric oxide films formed on the alloys are measured in a similar way to that mentioned above.

It has been discovered according to the invention that the dielectric oxide films formed on the alloy of titanium and zirconium in compositions shown in Table 1 can form a so-called solid electrolytic capacitor superior in leakage factor when combined with a manganese dioxide layer integrated thereon. Other semiconductive materials such as PbO NiO, SnO Cu O and C00 do not produce a superior solid electrolytic capacitor. Said manganese dioxide layer can be applied to said dielectric oxide film in a manner per se well known by a prior solid capacitor preparation technique. For example, an alloy according to the invention is anodically oxidized in a manner similar to that mentioned above and is immersed in a manganese nitrate solution and is heated at about 250 C to provide a manganese dioxide layer thereon.

Said manganese dioxide layer is coated with a carbon film layer by applying carbon ink containing colloidal carbon. Finally a silver paint as a counter electrode is applied to said carbon film layer in a per se conventional method.

Table 4 shows the electrical properties of solid electrolytic capacitors which have non-porous sintered bodies. The sintered bodies are made from titanium particles and zirconium particles in the aforesaid way. Additionally, the sintered bodies are heated in a vacuum and cooled to room temperature (about 30 C) from a temperature of 1,000 at a rate of 50 C per minute. The capacitance and the dissipation factor are measured by means of a capacitance bridge. The

Table 4 indicates the capacitance, dissipation factor and leakage of so-produced solid electrolytic capacitors as a fimction of the composition of the titaniumzirconium alloy. It will thus be readily understood that the alloy of titanium and zirconium rapidly cooled produces a so-called solid electrolytic capacitor having a low leakage factor.

A porous sintered body of titanium-zirconium alloy can be made by sintering a compacted titanium-zip conium alloy powder or a compacted mixture of titanium particles and zirconium particles in finely divided powdered form at a temperature of 800 to l,250 C in a non-oxidizing atmosphere.

Various mixtures of titanium and zirconium particles in compositions listed in Table 5 are pressed atabout 1,850 k g/cm into a tablet of 3 mm diameter and 3 mm height. The average grain sizes of used titanium particles and zirconium particles are about 60 and 4 microns, respectively. Each pressed tablet is provided with a film-forming lead wire made of niobium and is heated at l,000 C at a reduced pressure of mm Hg of air for 1 hour. The heated body is furnace-cooled to 1,000 C and is rapidly cooled to about room temperature (30 C) at a cooling rate of 50 C/minute by moving the heated body to an inner place of the furnace, which is cooled by a water pipe attached to the outer furnace. The sintered body has a porosityof about 30 percent and is anodically oxidized in a mannersimilar to that mentioned above.

The electrical properties of anodic oxide films on the porous sintered bodies mentioned above are listed in Table 5. The electrical properties are measured in a 10 percent phosphoric acid solution in a way similar to that described above.

TABLE 5 Composition Capacitance D.F. Leakage Factor (uA/uFN) wt.% at.% f/cm) (36) at at at Zr Zr 10V 20V 40V The anodized porous sintered bodies are put into a container filled with an electrolytic solution consisting of aqueous phosphoric acid solution, aqueous sulfuric acid solution, aqueous ammonium borate solution or aqueous potassium nitrite solution, and sealed with a plug to prevent the electrolytic solution from leaking. Construction of a so-produced so-called wet electrolytic capacitor is shown in FIG. 1.

The dielectric oxide film formed on the surface of said porous sintered body is coated with a manganese dioxide layer by the aforesaid method in accordance with the invention. The sintered body having a dielectric oxide film and a manganese dioxide layer is fabricated into a solid electrolytic capacitor having a fiif ii iid i ioiync capacitors comprising a porous anode have a leakage factor of 0.009 to 0.1 uA/uRV at an applied voltage of 15V in the zirconium composition range of 10 to 60 atomic percent.

The above description has been made with reference to an anodic oxidation method by an aqueous solution. However, the noveleffect of the alloy can be achieved when the alloy according to the invention is anodically oxidized in a fused salt of sodium nitrate, potassium nitrate, sodium nitrite or potassium nitrite. Among these, the preferable salt is sodium nitrate which is heated at a temperature of 310 to'400 C.

What is claimed is:

1. A method of making an electrical capacitor which comprises heating an alloy consisting essentially of 40 to atomic percent of titanium and 10 to 50 atomic percent of zirconium at a temperature of 500 to 1,000 C in non-oxidizing atmosphere, cooling the heated alloy to room temperature at a rate higher than 50C per minute, and anodically oxidizing the cooled alloy to form a dielectric film thereon.

2. A method of making an electrical capacitor according to claim 1, wherein said cooled alloy is anodically oxidized in a fused salt of sodium nitrate at a temperature of 310 to 400 C.

3. A method of making an electrical capacitor according to claim 1, wherein said cooled alloy is anodically oxidized in an aqueous phosphoric acid solution of 0.1 to 20 percent by weight concentration.

4. A method of making an electrical capacitor according to claim 1, wherein said alloy is a porous body prepared by heating a mixture consisting essentially of 40 to 90 atomic percent of titanium powder and 10 to 60 atomic percent of zirconium powder at a temperature of 800 to 1,250 C in a non-oxidizing atmosphere. 

2. A method of making an electrical capacitor according to claim 1, wherein said cooled alloy is anodically oxidized in a fused salt of sodium nitrate at a temperature of 310* to 400* C.
 3. A method of making an electrical capacitor according to claim 1, wherein said cooled alloy is anodically oxidized in an aqueous phosphoric acid solution of 0.1 to 20 percent by weight concentration.
 4. A method of making an electrical capacitor according to claim 1, wherein said alloy is a porous body prepared by heating a mixture consisting essentially of 40 to 90 atomic percent of titanium powder and 10 to 60 atomic percent of zirconium powder at a temperature of 800* to 1,250* C in a non-oxidizing atmosphere. 